The present invention pertains to adhesive compositions, particularly adhesive compositions for paper and business forms which normally exhibit no tack but which, upon application of pressure, exhibit normal adhesive properties.
Conventional adhesive coatings for paper products and business forms are generally very pressure sensitive or tacky. As a result, these coatings can attract and hold dust and dirt thereby loosing their adhesive properties. In addition, because these adhesive coatings are very pressure sensitive, paper products containing these coatings can be difficult to handle and process. In certain instances the prior art adhesive coatings inadvertently cause adjacent sheets of paper to stick together during processing. For example, business forms being processed from continuous rolls must have the ability to release from the opposite paper backing without transfer of the adhesive to the backing or tearing of the backing.
Adhesive coatings should also be relatively invisible so as not to distract from the appearance of the paper product. A business form adhesive should also remain intact when the business form is folded or handled. This requires a flexible coating that is color compatible with the paper substrate.
The prior art has attempted to overcome certain of these defiencies by encapsulating the adhesive in pressure rupturable capsules or in microscopic, closed cells. See for example Eichel, U.S. Pat. No. 2,907,682; Eichel U.S. Pat. No. 2,988,461; Brochman U.S. Pat. No. 3,565,247; and Marenelli U.S. Pat. No. 3,639,137. These techniques, however, require additional manufacturing costs associated with the encapsulation of the adhesive, or the attachment of the encapsulated adhesive to the substrate and are thus otherwise generally unsuitable for use in conjunction with the facile, economical production of paper products, particularly business forms paper products.
Levens U.S. Pat. No. 4,223,067, discloses a foam-like pressure sensitive adhesive tape comprising about 20-65 volume percent glass microbubbles uniformly dispersed throughout an adhesive matrix. The Levens tape has an adhesive thickness of 0.2 to 1.0 mm or more and can be covered by a low adhesion protective web. In addition, Levens indicates where strong bonds are required, a layer of unfilled pressure-sensitive adhesive should be applied on the microbubble filled adhesive. This structure is obviously not suited for use as an adhesive for business forms where a protective web on an unprotected pressure-sensitive surface is not desired.
Erwin, U.S. Pat. No. 3,314,838, discloses a pressure sensitive adhesive comprising a thin surface layer of adhesive over a monolayer of fragile, thin walled, hollow spheroidal particles, which in turn are in direct contact with a paper substrate. This structure fails to produce a pressure sensitive adhesive band suitable for use in business forms since it is activated by very low threshold pressure, e.g., hand pressure applied through a narrow squeegee or paper hangers' roller and is somewhat tacky to the fingers.